Evaluation of Handy GPSs and Their Applications
Shiro Ochi
Institute of Industrial Science, University of Tokyo
7-22 Roppongi, Minatoku Tokyo, Japan, 106
Tel: +81-3-3402-6231(2562) FAX :+81-3-3479-2762
E-mail : ochi@shunji.iis.u-tokyo.ac.jp
Abstract
Due to the expansion of GPS users, various types of GPS have appeared in the market. Accuracy is very much required for precise surveying, where cost performance is more important for many purposes such as car navigation system, field check investigation. The accuracy of hand GPS marketed for hobby use, named "Germin GPS-38" was examined in this paper. The error ranges of X(longitude). Y (latitude) and Z(altitude) direction are respectively -38 to +52 m, -130 m to +74 m. and -289 m to +71 m by single GPS. In order to improve the accuracy, pseudo differential method is effective, where the error ranges of X, Y and Z direction are -40 m to +49 m, -31m to + 33m, and -66m to +46m respectively. By averaging 10m minutes data at the same point with pseudo-differential method, 14m distance error in X-Y plain is acquired, and it is practical for ground truth investigation with Remote Sensing data.
1. Introduction
The number of GPS users is increasing rapidly in the field of not only surveying but also various purposes such as car navigation system. Because the cost of small systems of handy GPS is a few hundred. US$, and they can available anywhere in the world, handy GPSs are useful for field checking for Remote Sensing data. However the errors of these GPSs are reported as about 100m(Tsuchiyaand Tshuji 1995), it will be taken care to discuss the accuracy in pixel resolution for R. S. data which is resolution is few decade of meters. In this paper, "Germin GPS-38" that is popular for hobby purposes was used for the error verification, and the methods to improve the accuracy are proposed. The characteristics of the GPS are small and light to carry, and the data can be transferred to PC through RS232C interface. The position information displayed on the machine monitor changes every two seconds, and it makes difficult to define the position. So the data acquiring and correction were made by series of received data for about 50 minutes.
2. Error by Single GPS
By receiving GPS data at a point where the extract position is known, the errors in X(longitude), Y(latitude) and Z(altitude) were checked. Fig.-1 shows the fluctuation of the received data for 50 minutes at the same point, where the X axis is time and Y axis is error in meters. The data fluctuates, but has not specific cycle. The error of Y(latitude) direction which is from -130 m to +74m (204 m ranges) is more than the error of X (longitude) direction which is from -38m to +52m (90m range). Fig-2 shows the error fluctuation of the received altitude by the GPS. The maximum error appeared after ten minutes from starting the observation, however the error became stable, that is -50m to +50m range, after 15 minutes from the starting the observation. It is said that the accuracy of altitude is very much influenced by the number of captured GPS satellite (Tomita and Matsuyama 1996), however this was not considered in this paper.
Fig 1 Error fluctuation for X(longitude) and Y(latitude)
Fig-2 Error fluctuation for Z(altitude)
The result by analyzing the raw received GPS data means that the position information which can be obtained at a point and a time is sometime true accidentally, but it will be wrong in most cases. The error is more than 100m in X-Y distance and 250 m in Z direction in some cases. So the method to improve the accuracy was suggested by averaging 5 minutes data, 10 minutes data and 30 minutes data. Fig-3 show the averaged error for 5 minutes, 10 minutes and 30 minutes in X-Y direction(horizontal distance). The averaged 5 minutes error is 13m to 69m (56 m range), the averaged 10 minutes error is 11m to 34m (23m range) and the 30 minutes error is 12 m to 25m (13 m range). This means that by observing long period such as 30 minutes makes the accuracy improved.
Fig-3 Averaged error of horizontal distance
